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Get clickin ... the 70s were cool, when you were in your twenties ... LOL

Last edited by DonSchap; 05-30-2009 at 04:57 PM.

Don Schap - BFA, Digital Photography A Photographer Is ForeverLook, I did not create the optical laws of the Universe ... I simply learned to deal with them.
Remember: It is usually the GLASS, not the camera (except for moving to Full Frame), that gives you the most improvement in your photography.flickr® & Sdi

It's not that ... just dotting the "t's" and crossing my 'i's" ... or something like that.

Don Schap - BFA, Digital Photography A Photographer Is ForeverLook, I did not create the optical laws of the Universe ... I simply learned to deal with them.
Remember: It is usually the GLASS, not the camera (except for moving to Full Frame), that gives you the most improvement in your photography.flickr® & Sdi

Oh Boy! Are you guys getting your knickers in a twist or what.
If you take a pic with an A900 camera and crop it in Photoshop, the DOF won't change.
That's all the crop mode does on camera. Ergo the DOF is the same.

lol that's what I've been trying to say. In looking at the A900's internals there's no physical change to shrink the visible sensor size.

Hey, I appreciate the effort ... Real time response to questions. That's my kind of forum!

Don Schap - BFA, Digital Photography A Photographer Is ForeverLook, I did not create the optical laws of the Universe ... I simply learned to deal with them.
Remember: It is usually the GLASS, not the camera (except for moving to Full Frame), that gives you the most improvement in your photography.flickr® & Sdi

Hmmm. I think there are some basic misconceptions in evidence. I'm not sure if I can easily elucidate but here goes.

First of all, DOF is not a property of the lens nor does the lens have two zones of focus (in-focus and out of focus). There is only ONE plane of focus and everything in front of and behind that plane is out of focus to a lesser or larger degree. It doesn't matter whether you stick that lens on an A700, an A900 or a cardboard box, the lens is a constant factor and performs the same job it was designed for regardless.

So how is DOF determined. Well, it's not an absolute mathematical constant; it requires a subective assessment of what is acceptably out of focus. Back in the days when digital was a treat in store and we actually printed on photographic paper, it became accepted that a 35mm neg enlarged to an 8x12 print and viewed from a distance of 15" by someone with average eyesight was as big as you could get before the thing was unacceptible blurry. All very subjective and not very technical hey? but now we have a starting point.

A word about Cicles of Confusion (CoC). Going back to the plane of focus, you know that a point of light from that plane will be rendered at the film (or sensor) plane as a point but everything in front or behind plane of focus will be rendered as a circle which gets progressively larger with distance. It turns out that when the CoC on the 8x10 print gets to 250 microns is the point at which the print, viewed by that hypothetical person of averge eyesight, is percieved to be unacceptably blurry. Seeing as the 35mm neg was enlarged by a factor of 8.46 to get an 8x12 print, we can divide 250 microns by the same amount to get get a CoC at the film plane of 30 microns (A900).

If you go through the same exercise with the 1.5crop sensor A700 you arrive at a CoC at the film plane of 20 microns. Without going into the detail you guys are smart enough to know that the the CoC has to be less to start with in order to obviate the greater enlargement required to get to 8x10.

I hope I've convinced you that DOF is a subjective assessment of the blur factor and not the lens per se.

Of course the whole relationship is complex because of the number of factors which affect it (sensor size, aperture, field of view) but what I can say with certainty is that, comparing an A700 and A900, if you crop an A700 size portion out of the A900 sensor (which is what the camera does in that mode), the two DOFs will be exactly the same. You're using the same lens and same size image so you get exactly the same DOF, and that's where we came in.

Last edited by Peekayoh; 05-31-2009 at 12:36 PM.
Reason: Mixing up my 8x12 with 8x10s

Hmmm. I think there are some basic misconceptions in evidence. I'm not sure if I can easily elucidate but here goes.

First of all, DOF is not a property of the lens nor does the lens have two zones of focus (in-focus and out of focus). There is only ONE plane of focus and everything in front of and behind that plane is out of focus to a lesser or larger degree. It doesn't matter whether you stick that lens on an A700, an A900 or a cardboard box, the lens is a constant factor and performs the same job it was designed for regardless.

So how is DOF determined. Well, it's not an absolute mathematical constant; it requires a subective assessment of what is acceptably out of focus. Back in the days when digital was a treat in store and we actually printed on photographic paper, it became accepted that a 35mm neg enlarged to an 8x10 print and viewed from a distance of 15" by someone with average eyesight was as big as you could get before the thing was unacceptible blurry. All very subjective and not very technical hey? but now we have a starting point.

A word about Cicles of Confusion (CoC). Going back to the plane of focus, you know that a point of light from that plane will be rendered at the film (or sensor) plane as a point but everything in front or behind plane of focus will be rendered as a circle which gets progressively larger with distance. It turns out that when the CoC on the 8x10 print gets to 250 microns is the point at which the print, viewed by that hypothetical person of averge eyesight, is percieved to be unacceptably blurry. Seeing as the 35mm neg was enlarged by a factor of 8.46 to get an 8x10 print, we can divide 250 microns by the same amount to get get a CoC at the film plane of 30 microns (A900).

If you go through the same exercise with the 1.5crop sensor A700 you arrive at a CoC at the film plane of 20 microns. Without going into the detail you guys are smart enough to know that the the CoC has to be less to start with in order to obviate the greater enlargement required to get to 8x10.

I hope I've convinced you that DOF is a subjective assessment of the blur factor and not the lens per se.

Of course the whole relationship is complex because of the number of factors which affect it (sensor size, aperture, field of view) but what I can say with certainty is that, comparing an A700 and A900, if you crop an A700 size portion out of the A900 sensor (which is what the camera does in that mode), the two DOFs will be exactly the same. You're using the same lens and same size image so you get exactly the same DOF, and that's where we came in.

Peek,
I understand your points. I agree we are talking about a plane or area of focus (DOF) where objects are subjectively sharp enough to be considered "in focus" and where objects in front of and behind the DOF are subjectively blurry enough to be considered "out of focus." I don't think we are talking about a matter of mathematical certainty for the size of the DOF plane/area given the factors of sensor size, focal length, aperature setting, and distance to subject, but rather an understanding of the role those factors play in the relative size of the DOF plane that is going to produce the subjectively "in focus" subjects that will be pleasing to the majority of the audience.

I think we all would agree that at the same f-stop setting a 50mm lens will produce a wider field of view and a correspondingly narrower DOF as compared to a 70mm lens which will produces a narrower field of view with a correspondingly larger DOF. My curiosity is whether the "crop factor" of the APS-C sized sensor is simply a matter of a reduction in the field of view or whether it also has an effect on the DOF. Hence the experiment. After all, if we knew the answer for sure, then I wouldn't be out today experimenting!!

Cropping with that switch does the same thing as cropping in Photoshop. Depth of field is only considered to be different on crops and full frames because the camera to subject to background distances are different for the same framing on a full frame camera versus a crop camera.

Self correction

Originally Posted by DWessel

Peek,
... I think we all would agree that at the same f-stop setting a 50mm lens will produce a wider field of view and a correspondingly narrower DOF as compared to a 70mm lens which will produces a narrower field of view with a correspondingly larger DOF. My curiosity is whether the "crop factor" of the APS-C sized sensor is simply a matter of a reduction in the field of view or whether it also has an effect on the DOF. Hence the experiment. After all, if we knew the answer for sure, then I wouldn't be out today experimenting!!

I misspoke and am self-correcting ...

I think we all would agree that at the same f-stop setting a 50mm lens will produce a wider field of view and a correspondingly deeper DOF as compared to a 70mm lens which will produces a narrower field of view with a correspondingly narrower DOF. ...

I would also add that if the APS-C function on the α900 just changes the crop factor, then about the only usefulness I can see for the function is if one had the Sony DT, Tamron D-II, or Sigma DC lenses which are optimized for the APS-C sized sensor.